At the peak of the dot-com bubble in the early 2000s, Nirav Tolia watched his consumer reviews website Epinions surge to a valuation of about $150 million, then quickly plummet to $30 million. As CEO, Tolia presided over layoffs, a merger and ultimately was named in a lawsuit from three of his cofounders alleging they were not properly informed about the company's financial situation.

"It was exceptionally difficult to go through," Tolia says now of the abrupt valuation changes. "What this experience teaches you is that valuation is temporary, and valuation does not equal value. "

More than a decade later, valuation is on his mind again, but under better circumstances. Nextdoor, a social network for neighborhoods that Tolia co-founded in 2010, announced raising $110 million at a $1.1 billion valuation. Employees marked the milestone by sharing an "internal email" showing the list of billion-dollar startups. The general sentiment among staff, according to Tolia: "We're flattered that we're in that list of companies."

That list has grown considerably longer in recent months as institutional investors and hedge funds have begun pouring more money into private companies in search of greater returns in a market climate of low interest rates. These startups, in turn, can use that ample funding to delay going public longer. And so it is that billion-dollar startups, once rare enough to go by the shorthand "unicorns" in the industry, have become more commonplace.

The Wall Street Journalputs the number of billion-dollar startups at 75; Fortuneestimates there are more than 80. CBInsights, a venture capital database, reports that 38 companies hit that milestone in 2014. This week alone, Nextdoor and Farfetch, a fashion-focused website, were added to that list.

While it may be less exclusive than it once was, membership in the billion-dollar startup club still has the perk of boosting a company's momentum and legitimacy. "From a recruiting standpoint and a press standpoint, there is definitely a perception that your company is in a different league," Tolia says.

"It does make it easier. It makes you seem more stable, and from a hiring perspective it probably makes you more visible, which I think is probably good. People want to work with companies that are winning," says Tom Gonser, founder and chief strategy officer at DocuSign, which secured a $1.6 billion valuation last year. After that, he received a personal reminder of how much people pay attention to the milestone when a friend in another country sent him a picture of a unicorn. "It's a global thing for sure."

That difference in perception is reason enough for some founders to make a deliberate push to hit the billion-dollar mark, sometimes in remarkably fast time.

Image: CB Insights

Stewart Butterfield, who previously cofounded and sold Flickr to Yahoo, began talking with investors last year about a possible funding round for his new company Slack even though it had plenty of cash in the bank. As he later explained, the round was contingent on Slack receiving a billion-dollar valuation. "It does make a difference psychologically," Butterfield toldFortune. Sure enough, Slack hit that 10-figure valuation — less than a year after launching.

Some prominent investors have raised concerns about this line of thinking, without naming any one company in particular. Chris Sacca, an investor in Twitter and Uber, was quoted at an event this week saying businesses are calling it "table stakes" to be valued at $1 billion or higher. "That's f*cked," he said. "That's completely f*cked."

Then there are those founders like Apoorva Mehta who watch with amazement as their startup's funding and valuation balloon at an unimaginably fast pace. Mehta went out looking to raise $150 million at the end of last year for Instacart, the grocery delivery service he launched in 2012. That soon turned into a $220 million round at a $2 billion valuation, amid strong investor demand.

"It's definitely overwhelming," Mehta admitted in an earlier interview with Mashable. "It seems like things are moving a lot faster than I guess I could ever have expected.

Gonser, the DocuSign founder, has occasionally questioned himself as his company's valuation and business have soared. "I’m by nature a relatively paranoid person [with thoughts like] what can possibly go wrong? And how can we go faster?" he says. "I think on scale, it's probably amped up my angst a little bit because it's so big."

For the most part, though, founders try to publicly frame the billion-dollar mark as just another number — and one that could rocket up or crash down based on factors they might not entirely be able to control, as Tolia from Nextdoor experienced years ago.

"It's a nice milestone, but nothing to celebrate," says Kevin Hartz, cofounder and CEO of Eventbrite, which reportedly joined the unicorn club last year, and who has invested in a host of multi-billion-dollar companies like Airbnb and Pinterest. "It's typically always secondary to what we’re trying to do with our customers, what we’re trying to do with our product and our teams and our business."

When asked whether he ever worries that there may be too many billion-dollar startups, Hartz argues that "the unicorns as a portfolio" are really "a fantastic portfolio." Then he adds a key caveat: "The public markets will be the ultimate judge of whether these values are fair or not."

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